At any stage in an optical path in, for example, an optical transmission system for an optical computing system, pulses used to transmit data may become distorted. It is most important that the information can be recovered from the optical signal at a receiver, or that the signal can be reconstructed at a repeater, for onward transmission. As bit rates increase, the expense and complexity of receiver and regeneration (or repeater) hardware rises considerably.
For retiming of optical pulses for multiplexing or regeneration, the basic requirement is for a device which will sample the signal accurately.
It is known to do this electrically at lower speeds, and various attempts have been made to do this optically at higher speeds. For example, a decision gate for all optical data retiming, operating at 1 Gbit/s is known from Electronics Letters Jan. 7, 1993, volume 29, no. 1 "Decision Gate for all Optical Data Retiming Using a Semiconductor Laser Amplifier in a Loop Mirror Configuration", Eiselt et al. A loop interferometer is fed with the raw data signal and a clean clock signal. The clock pulses are modulated by the data using a semiconductor laser amplifier in the loop. This shifts the phase of the clock signal, when data is present, so as to cause constructive or destructive interference. Thus the clock signal can be output with its amplitude modulated by the data. This effectively means that the modulated clock can be regarded as a regenerated, retimed data signal. The document discusses the application of this technique in an all optical regenerator, though no disclosure is given of how the clock could be derived from the data signal, for use in the loop interferometer.
Another regenerator using a Sagnac interferometer, which is a type of loop interferometer, is shown in Electronics Letters, Jul. 2, 1992, volume 28 , no. 14 "All Optical Regenerator Based on-Non-Linear Fibre Sagnac lnterferometer", Jinno et al. Again, timing and amplitude restoration is carried out on the data stream, but no disclosure is given of how the clock might be created at the regenerator.
A problem with such loop type interferometer samplers or regenerators, is that they turn out to be impractical in real systems because the bit rate is fixed by the geometry of the devices, because they are difficult to control to achieve good results in practice, and because they would be difficult to integrate.
Another use of interferometer structures has been proposed for optical switching, eg for optical time division multiplexing systems. All optical demultiplexing and add-drop multiplexing is disclosed in "Topical meeting on Optical Amplifiers and their Applications", Jul. 11-13, 1996, Monterey, Calif., "Optical Signal Processing Using Monolithically Integrated Semiconductor Laser Amplifier Structures", Agrawal et al. A semiconductor optical amplifier is used as a phase shifter in one arm of a two arm interferometer. The data stream to be switched is fed to both arms of the interferometer. A control signal, eg a regular pulse train for demultiplexing, is fed into one arm. Constructive or destructive interference is caused when the signals in the two arms are combined. Such an arrangement is shown in FIG. 1. Optical amplifiers 1, 2 are provided in each arm.
It is also known to sample optical signals using a crystal, to make eye-diagram measurements, from the 22nd European Conference on Optical Communication, 1996, Oslo, "Eye-diagram Measurement of 100 Gbit/s Optical Signal Using Optical Sampling", Takara et al. A high power sampling pulse at 200 watts and with pulse lengths of less than half a picosecond, were fed into an organic non-linear crystal. The non-linear characteristic of the crystal enables the data signal to be sampled optically, though the high powers required make the system impractical for commercial use.
It is also known to use a two arm interferometer such as a Mach-Zehnder interferometer for conversion of wavelength of a data stream and simultaneous signal regeneration, from IEEE Photonics Technology Letters, volume 8, no. 9, September 1996, "Gbit/s Wavelength Conversion with Integrated Multi Quantum-Well-Based Three-Port Mach-Zehnder Interferometer", Idler et al. As shown in FIG. 2, all optical wavelength conversion with simultaneous shaping is achieved by supplying a two arm interferometer with a continuous wave input. This is modulated by the data to produce a wavelength converted data output signal. Optical amplifiers 10 and 11 in each arm of the interferometer provide the necessary phase shift according to the data input. No retiming occurs.
There remains a need for a practical optical sampling device.